The use of positron-labeled tracers for the diagnosis, staging and monitoring of various diseases using, for example, Positron Emission Tomography (PET) has grown in use over the past 20 years. Examples of some widely used, clinically approved imaging agents include [F-18]FDG, [F-18]NaF and [F-18]Fluoro-DOPA. Several other tracers, while not clinically approved, are being used for investigational purposes. Examples of these tracers include [F-18]FLT and [F-18]AV-45.
Typically, the radiosynthesis of positron-radiotracers are performed remotely behind lead-shielded containers using automated, semi-automated or manual synthesis instrumentation. Large amounts of lead shielding are required to protect the operators from radiation emitted by the labeling isotopes. As a consequence, all of the hardware, reagents and plumbing must be prepared and readied prior to the introduction of radioactivity into the synthesis platform. Unprotected interventions by the operator into areas containing large amounts of radioactivity for the purposes of making hardware adjustments, addition of reagents or facilitating the transfer of materials are heavily discouraged.
For routine syntheses, the need for operator intervention is minimized because the chemistry is well supported by the instrumentation's hardware. For example, the synthesis of well-established radiotracers follows a well-defined protocol of isotope activation (eg. preparing anhydrous [F-18]fluoride), labeling, deprotection (if required), purification and formulation for injection. Such well known protocols are used for making several common tracers known in the art such as [F-18]FDG, [F-18]FLT, [F-18]FHBG and [F-18]MISO.
In general, a drawback of the synthesis of these radiolabeled molecules is undoubtedly time consuming, labor intensive and randomly unreliable. In addition, highly complex and expensive instrumentation is required for successful production of these tracers. In an effort to minimize production and cost issues, radiochemists have attempted to reduce the labeling procedures to their simplest, quickest and most reliable protocols on mechanically simple platforms. Despite these process improvements, radiolabeling protocols still contain inherent inefficiencies that would benefit from further chemistry, process and hardware improvements.
Currently available platforms designed to produce radiolabeled compounds are typically complex and inefficient, which are not suitable to fit for newly developed processes. For example, PCT/US11/31681 describes a procedure where a solvent with a predetermined amount of water in at least one organic solvent is used to a) elute the 18F-fluoride from an anion exchange cartridge and h) perform the 18F-labeling, without drying the 18F-fluoride, in the presence of at least one labeling reagent and at least one phase transfer catalyst. In other methods, the 18F-fluoride solution may be dried. The incorporation of hydrous labeling protocols, such as that described in PCT/US11/31681, is not easily transferable to existing synthesis platforms for the reason that these platforms were not designed for such protocols. For example, for anhydrous protocols, existing apparatuses comprise many unnecessary components. Fluid paths and valves are superfluous and provide little or no benefit in supporting the chemistry.
Conventional platforms for making FDG have several features in common as their processes follow well-established protocols. To begin with, anion exchange cartridges sequester [F-18]fluoride ion from [O-18]water. The trapped [F-18]fluoride ion is then released from the anion exchange cartridge into a reaction vessel. A typical anion exchange cartridge may include a Quaternary Methylammonium Anion exchange cartridge (QMA). The vessel is then heated under a stream of an inert gas such as nitrogen to remove the excess moisture. Once the moisture is sufficiently removed, then a labeling precursor is added, typically as a solution in an organic solvent. The reaction mixture is then heated for a period of time to induce the nucleophilic addition of [F-18]fluoride onto the labeling precursor. After this labeling is finished, there is an optional series of post-labeling processing steps which may include a deprotection and or neutralization step. The contents of the reaction vessel are then transferred out for post-labeling purification. For simple purifications, a series of small cartridges are used to remove unwanted by-products, adjust pH and render the dose suitable for injection. For more complex purifications, typically reverse-phase High Performance Liquid Chromatography (HPLC) purification is performed with an optional reformulation of the isolated HPLC fraction to render the dose suitable for injection. There are variations between different types of synthesis platforms at each step of the production protocol. For instance, some platforms employ vacuum to assist in the evaporation of moisture during the [F-18]fluoride drying step, while other platforms simply use inert gas without the aid of negative pressure. However, methods and hardware employed for preparing this moisture-free [F-18]fluoride are not necessary.
The conventional means for 18-labeling involves the formation of “activated” or “naked” fluoride, i.e. fluoride that is sufficiently moisture-free and thus suitable for radiolabeling. It is widely known that the desolvation of fluoride increases its nucleophilic character. See V. M. Vlasov, “Fluoride ion as a nucleophile and a leaving group in aromatic nucleophilic substitution reactions”, J. of Fluorine Chem., vol. 61, pp. 193-216 (1993). In these conventional labeling protocols, trace amounts of 18F-fluoride are sequestered onto an anion exchange column from several milliliters of 18O-water. Afterwards, the 18F-fluoride ion is eluted from the anion exchange column through the use of salts, such as K2CO3, dissolved in water. An additive/catalyst such as the potassium crown ether Kryptofix™ K222, which is dissolved in anhydrous acetonitrile, may be used in conjunction with aqueous K2CO3 to facilitate the elution of 18F-fluoride, or optionally added into the reaction vessel after the K2CO3-mediate elution.
After the elution step, there is an extensive drying protocol needed as reagents K2CO3 and Kryptofix™ K222 are in a highly hydrous solution of acetonitrile. This drying step generates an activated mixture of K2CO3, Kryptofix™ K222 and 18F-fluoride. The drying process begins by evaporating the azeotropic mixture at elevated temperatures, oftentimes at reduced pressures to aid in the evaporation of water from the reaction vessel. This initial drying can take up to 30 minutes to complete, depending on the efficiency of drying. After the first evaporation, it may be necessary to perform additional evaporations to effectively remove of enough water to render the 18F-fluoride sufficiently moisture-free for labeling.
There are several inherent problems with this approach to generating activate reagents for 18F-fluorination. First, the amount of residual water after the initial drying step may vary from run-to-run given mechanical differences in vacuum, gas flows, valve integrity and temperature control. Mechanical problem, either a single one or combination of a few, will negatively impact the efficiency of drying and hence, the labeling results. Since the amount of residual water could vary greatly from run to run, the radiolabeling results would then be inconsistent, making reliable production of radiotracers difficult. Also, given the time needed to successfully dry the fluoride, a good portion of the total synthesis time is dedicated to the drying step. Lastly, because of the concern of residual water in the reaction, there is a potential for operators to “overdry” the reaction mixture prior to fluorination. In this instance, drying the reagents for too long may be as equally hurtful as under-drying the reagents (under-drying being the failure to remove sufficient moisture from the reagents for 18F-fluorination). For example, Kryptofix™ K222 decomposition is directly related to drying times and temperatures: prolonged drying at high temperature compromises the integrity and functionality of Kryptofix™ K222. To address these issues, a method that minimizes the length of drying and can accurately control the amount of moisture from run to run would be a substantial improvement to current radiolabeling practices.
Alternate methods have been developed in an attempt to obviate the need for the drying step that either elute 18F-fluoride from anion exchange resins using additives in either anhydrous organic solvents (such as acetonitrile, see Joel Aerts et al., “Fast production of highly concentrated reactive [18F] fluoride for aliphatic and aromatic nucleophilic radiolabeling”, Tetrahedron Letters, vol. 51, pp. 64-66 (2009); International Patent Application Pub. No. WO 2009/003251) or by using ionic liquids in hydrous acetonitrile (Hyung Woo Kim et al., “Rapid synthesis of [18F]FDG without an evaporation step using an ionic liquid”, Applied Radiation and Isotopes, vol. 61, pp. 1241-1246 (2004)). For these types of elutions using compounds with unknown toxicities, one would want to assay for these additives in the final product prior to injection and imaging, which ultimately complicates the production workflow.
Efforts to increase the yield of the synthesis have also been reported. The use of hydroalcoholic (i.e. protic solvents) co-mixtures is reported to improve 18F-labeling yields over the standard single solvent IT-labeling conditions. Dong Wook Kim et al., “A New Class of SN2 Reactions Catalyzed by Protic Solvents: Facile Fluorination for Isotopic Labeling of Diagnostic Molecules”, J. Am. Chem. Soc., vol. 128, no. 50, pp. 16394-16397 (Nov. 23, 2006). While the increases in yields are believed to be a result of the unique interactions between the 18F-fluoride and possibly the leaving group on the precursor, it is not practical to use hydroalcoholic solvents, such as t-amyl alcohol, as they must be analyzed in the final product. Additionally, the low polarity of these bulky solvents can hinder the precursor's solubility which can be used for the labeling reaction, thus negatively impacting the radiolabeling yield.
In addition to the complicated activation of [F-18]fluoride, existing synthesis platforms often utilize expensive components that require constant cleaning from run-to-run. This process may consume additional reagents, time and lifespan of the individual components. Most systems require regular (e.g., quarterly) maintenance, which is costly and takes the machine out of operation for a period of time. Removal of this cleaning protocol has several distinct advantages.
Moreover, the field of PET chemistry is also always evolving with the development of new tracers. These tracers can possess more efficient imaging characteristics than previous agents or may target novel diseases or biochemistry not previously known or targeted. With the advent of new tracers, new chemistries often evolve without the hardware supporting the preparation of these new tracers. Therefore, it is common for developmental runs to be performed at low levels of radioactivity with several manual operations needed in order to find a suitable protocol for production.
Oftentimes, new chemistries are often utilized without the convenience and safety of dedicated hardware. As an example, the synthesis of galacto-RGD is shown in “[18F]Galacto-RGD: Synthesis, Radiolabeling, Metabolic Stability, and Radiation Dose”, Haubner et al., Bioconjugate Chem. 2004, 15, 61-69. Estimates were not easily automated requiring many manual interventions. As a result, there is in an increased risk for radiation exposure to the operator during these intervening operations. As a second example of new chemistries not supported by current instrumentation, the increased use of click chemistry oftentimes occurs on synthesis platforms not fully equipped to support the necessary transfers of radioactivity, again increasing the risk of radiation exposure to the operator during the intervention steps.
Therefore, there is a need for systems that support new radiolabeling chemistries in a safe and efficient manner. There is also a need for a synthesis module that is inexpensive to manufacture, efficient, free of unnecessary components, reliably produces labeled tracers and has components that are easily disposable. For example, simpler and more efficient designs are desirable to support efficient and simple radiolabeling protocols such as those described in PCT/US11/31681